MOD 8 Flashcards
Mohr’s method measures the concentration of ions…
Cl-, Br- and CN-
Volhard’s method measures the concentration of ions…
Cl-,Br-,CN-,CrO2-4, S2-, PO43-,CO32-
Describe Mohr’s method of determining the concentration of the ion in question
Mohr’s method involves a direct titration with silver nitrate
- Suppose we have an unknown concentration of chloride ions in a flask, We can deliver via a burette, a solution of excess silver nitrate which will precipitate all the Cl ions to form AgCl ppt.
- To identify the endpoint, a potassium chromate indicator is added to the flask at the beginning so that as a second precipitate forms between the excess Ag being added and the Chromate ions in the flask, a new colour change occurs (yellow to blood red) indicating the endpoint.
The mean titre helps to determine the endpoint marked by the colour change, knowing the average titre we can do titration calculations as usual.
What are the limitations of Mohr’s method?
- If multiple ions are present in the analyte, we don’t get great results, Mohr’s method only works best when we know only one precipitate ion is in the analyte
- Colour change does not occur at the true equivalence point, Instead, a colour change occurs when excess silver reacts with chromate and forms a deep red solution, at this point all chloride, bromide and cyanide ions have been ppt out. Can overcome this by performing blank titration, (without analyte) This gives the titration error which can be subtracted from the endpoint to find the true equivalence point.
Describe in detail Volhard’s method to determine the concentration of the ion in question
Involves a back titration to analyse concentration.
- A known excess amount of silver nitrate is added to an unknown sample containing one of the above ions. Given that an excess of silver nitrate is added, all the anions will precipitate and there will be a (known) excess of silver ions remaining. The precipitate is filtered out, leaving the excess ions alone in the flask.
- The excess silver can be titrated against a standard solution of potassium thiocyanate, producing AgSCN(s)
- Fe3+ ions would have been added at the start to the solution, so that when SCN- ions precipitate out all of the silver ions, they will then react with the Fe3+ ions to form a blood-red solution of Fe(SCN)2+
The production of Fe(SCN)2+ and the colour change is the indicator that the endpoint has been reached
Limitations of Volhard’s method
As it is a back titration since the thiocyanate is added to the same reaction vessel as the AgX had formed if the AgX is more soluble than AgSCN, the AgX ppt. Will redissolve : AgX + SCN -> AgSCN + X-
So for the SCN to start reacting with the Fe3+ it would first have to mop up all of the redissolved AgX, then the excess Ag+ from the initial titration, and then only can it react with the Fe3+
Therefore if the Ksp of AgX > Ksp of AgSCN the AgX will re-dissolve, so it needs to be filtered out before titration with SCN-
What are the steps to conduct a flame test?
- Clean a nichrome wire (inert and high MP) by firstly dipping it in distilled water then 6M HCl and heating over a bunsen burner flame to vaporise any impurities on the wire
- Dip the clean nichrome wire into chloride solution (these are the most volatile) then when containing ions to be tested, the wire is held in the blue flame ⅓ from the base (hottest area of flame). Observe and record flame colour
- Re-clean the wire by repeating step 1 (to avoid cross-contamination)
- Repeat steps 1-3 for all other ions
What are the steps to conduct gravimetric analysis?
- Weigh the initial solution
- Add the precipitating solution in excess or as required
- Filter out the precipitate making sure to dry it completely
- Weigh the dry precipitate on electronic balance
- Find the number of moles of the precipitate and in so doing, calculate the moles of the ion in question, use the moles value to calculate either a required mass or concentration
What is the Haber process and justify the chosen reaction conditions
N2(g) + 3H2(g) -> 2NH3(g) H=-92kJ/mol
High temperatures are desired to increase the reaction rate however the reaction is exothermic and this would decrease yield as per LCP. Thus, a moderate temperature is chosen (400-450 degrees Celsius)
High pressures are desirable as this would increase yield as per LCP however high pressures can be unsafe (explosion risk) and costly to maintain. Thus a moderate pressure is chosen (200 atm)
An iron oxide catalyst is used to increase the reaction rate, this is not only profitable but may also be leveraged to use a lower temperature and increase yield whilst keeping the reaction going at an industrially profitable reaction rate
The removal of ammonia once formed, and recycling of reagents pushes the equilibrium to the right, maximising yield.
What are the common considerations in designing a chemical synthesis?
High Temperatures and pressures can be unsafe and also expensive to maintain
Specialised equipment and greater energy are required to create and maintain high pressures
Side reactions are more likely at higher temperatures
Reactants, products and catalysts often decompose at higher temperatures
Location of production plant: should be close to where materials are sourced but far from urban cities.
What are the economic considerations of designing chemical synthesis?
JOBS:
- The chemical manufacturing industry contributes largely to the Australian economy, creating many jobs and stimulating economic growth in positive ways.
IS PRODUCT PROFITABLE:
- Industrial uses must be considered – e.g. if demand is low, it may not be profitable to produce.
TRANSPORTATION COSTS:
- Intended industrial uses of a product will impact the location of the chemical production plant, for e.g. reduce transportation costs
Other considerations: cost of waste management and cost of raw materials and land.
What are the environmental considerations of designing chemical synthesis?
Enhanced greenhouse effect:
- Burning of fossil fuels releases harmful greenhouse gases into the atmosphere,
and accelerates climate change, burning of minerals containing sulphur (SOx) or
nitrogen (NOX) can lead to acid rain, which corrodes buildings and defoliates trees
Waste management:
- Solid waste is often disposed into the ocean which is harmful to ecosystems
What are the social considerations of designing chemical synthesis?
IMPROVED STANDARD OF LIVING:
The products of chemical manufacturing have increased our standard of living.
The use of plastics allowed for the creation of everyday objects, like toothbrushes.
Development of fuel which is used in combustion engine cars has improved our ease of transport. Pharmaceuticals have increased the standard of living by way of medicines.
What is an example, of the need for environmental monitoring?
Chlorofluorocarbons (CFCs) are chemicals that were previously used in deodorants and refrigerants. However, many countries stopped using CFCs in their products once environmental monitoring and chemical analysis revealed that CFCs were destroying the ozone layer. The chlorine atoms released from CFCs can react with ozone, turning ozone into oxygen gas
chlorine + ozone → chlorine monoxide + oxygen gas CFCs were therefore found to deplete the ozone layer because a lot of ozone (O3) was becoming oxygen gas (O2). As a result, CFCs are now banned in most areas to protect our ozone layer and human health. A suitable replacement for CFCS in terms of reducing their impact is hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs)
How can a mass spectrometer be used to analyse the structure of organic compounds?
(Only charged ions can be detected, as unlike neutral species, charged ions can be influenced by magnetic fields)
1. Ionisation - A gaseous vaporised sample is injected into the
ionisation chamber. Sample bombarded with high energy
electrons to form radical cations, many of which are unstable and fragment into ions and radicals
2. Acceleration – radical cations are accelerated using 2 metal plates and the electric field between them, uncharged particles cannot be accelerated
3. Electromagnet – positive ions are deflected via a magnetic
field, the amount of deflection depends on the mass-to-charge
ratio (m/z), smaller particles deflect more, thus by varying
the magnetic field, different m/z are able to strike detector
4. Detection – measures the intensity and outputs a graph,
indicating the relative abundance of ions present.
